1. Field of the Invention
The present invention relates to quaternary ammonium hydroxide-containing polymers and to their use in cationic electrodeposition. More particularly, this invention relates to the use of quaternary ammonium hydroxide-containing polymers in controlling the acid content of an aqueous electrodeposition bath which contains an acidified cationic resinous coating vehicle. The invention also relates to the use of quaternary ammonium hydroxide-containing polymers as coating vehicles for cationic electrodeposition.
2. Brief Description of the Prior Art
Cationic electrodeposition as practices on an industrial scale involves immersing a metallic article in an aqueous dispersion of a polymeric material which contains cationic groups such as quaternary ammonium salt groups or amine salt groups. An electric current is passed through the dispersion between the metallic article as cathode and a counter electrode as anode to cause a deposition of the coating on the cathode. During the electrodeposition, acid is generated at the counter electrode and continuously builds up in the bath where it can corrode equipment and raise the conductivity of the bath making it more difficult to electrodeposit smooth uniform coatings.
The art has recommended numerous ways to control this acid build-up such as by subjecting the bath to ultrafiltration and electrodialysis. However, these methods have shortcomings associated with them in that they require additional equipment costs as well as the expense of monitoring and maintaining the additional equipment.
The present invention overcomes many of the disadvantages of the prior art. In the present invention, the acid build-up is controlled by adding a quaternary ammonium hydroxide-containing polymer to the electrodeposition bath. The quaternary ammonium hydroxides are strong bases and react with the acid to form the corresponding quaternary ammonium salt of the acid. Thus, fresh resinous vehicle is provided and the pH of the bath is controlled and maintained at acceptable levels with no additional equipment costs.